1. Field of the Invention
The invention relates to an apparatus for liquefying a thermoplastic high-polymer material comprising a supply container for the thermoplastic material, heating means for heating the thermoplastic material in the supply container, a base part arranged beneath the supply container and containing a collecting trough for the liquefied thermoplastic material, and control means for regulating the heating means for the supply container in dependence upon the result of a comparison between a given desired value and an actual value detected by a sensor.
2. Description of the Prior Art
Such a liquefying apparatus is known, for example, from German patent specification 2,836,545 (corresponding to U.S. Pat. No. 4,308,447) or 3,600,020 (corresponding to U.S. Pat. No. 4,724,983) and comprises a supply container for the thermoplastic high-polymer material, heating means for heating the thermoplastic material in the supply container and a collecting trough for the liquefied material arranged beneath the supply container, said material being supplied from said trough to further processing, for example an applicator. In the supply container, which can also be referred to as "premelt zone", a relatively exactly defined temperature of the high-polymer material must be maintained to avoid unfavorable influences on the further processing; said premelt temperature is as a rule about 20 to 30% below the final temperature of the liquefied thermoplastic material in the collecting trough.
To maintain this temperature in the premelt zone, which is essential for the further processing, temperature sensors are provided either in the walls or in the bottom of the premelt zone detecting the actual temperature in this region and then supplying said temperature to a control means comparing the actual value of said temperature with a predetermined desired value, so that a corresponding adjustment of the heat supply to the premelt zone can be made.
It has, however, been found that such a temperature control cannot follow rapidly enough the fluctuations of the operating conditions occurring so that, as a rule, excessive deviations from the predetermined desired values arise.
As an alternative, attempts have already been made to provide a temperature sensor in the liquefied mass in the premelt zone, for example beneath the bottom of the supply container, said sensor detecting directly the temperature of the mass of the thermoplastic high-polymer material. However, once again delays in the response of the temperature detection occur which are due essentially to the poor thermal conductivity of the thermoplastic high-polymer materials and the resulting very inhomogeneous temperature distribution.